Lenses and microlenses are useful in a variety of contexts. Such contexts include, for example, coupling a light beam from a III-V semiconductor light emitting diode (LED) to an optical fiber, as well as coupling a light beam from an optical fiber to a III-V semiconductor optical detector, such as an avalanche photodetector (APD). Such contexts also include coupling a light beam from one fiber to one other fiber or to a plurality of other fibers. For use in some other contexts, the lenses are metallized (coated with a thin metallic layer), to form mirrors.
A method of fabricating InP (indium phosphide) microminiature lenses is described in a paper entitled "Ion-Beam Etching of InP and Its Application to the Fabrication of High Radiance InGaAsP/InP Light Emitting Diodes" by O. Wada, published in J. Electrochem. Soc.: Solid-State Science and Technology, Vol. 131, No. 10, pp. 2373-2380 (1984). In that method, first a photoresist layer was deposited on a major surface of an InP substrate. Then the photoresist layer was patterned solely into circularly cylindrical island(s) by conventional photolithographic techniques. The resist was then uniformly exposed to UV radiation. Next, the patterned photoresist was baked at a sufficiently high temperature so that each island was transformed into an island having a dome or spherical contour whose shape was determined by the perimeter of the island and by surface tension. Then the substrate was subjected to Argon ion-beam etching, the beam being oriented at an angle .theta. with respect to the normal to the major surface of the substrate. During the etching, the substrate was rotated about an axis along a radius of the spherical contour perpendicular to the major surface of the substrate. In this way, the final result was an (elliptically) aspheric microlens formed in the substrate.
The microlenses thus obtained in the above-described method suffered from undesirable surface roughness (except for those portions of the spherical surface whose normals were oriented at angles of approximately 65.degree. with respect to the ion beam). Although such roughness might be ameliorated by wet etching, such a procedure would entail an extra step that would require careful control over the parameters of the procedure. Moreover, the ratios of diameter to focal length of microlenses obtained in the above-described method are not as high as desired for some practical applications. Also, because of the non-unity ratio of etch rates k (as used on p. 2378 of the above-mentioned paper) of the substrate to the photoresist mask, only prolate elliptically aspheric microlenses, with only one axis of symmetry perpendicular to the substrate surface, are obtainable in the above-described method. However, in some applications spherical lenses, oblate elliptically aspheric lenses, or astigmatic aspheric lenses having two axes of symmetry in the substrate surface are desired, such as in systems where a semiconductor edge laser is to be coupled to a circular fiber by a cylindrically-corrected microlens. Finally in some contexts--particularly where the lenses are to be protected against metallic or other features that are ultimately desired for device function--it is desirable that the lenses or mirrors be fully recessed from the substrate surface, or at least partially recessed, for mechanical protective purposes. As used herein, the term "partially recessed" refers to a situation in which each lens is surrounded by a layer of material located on the substrate and having a thickness (height above the substrate) which is equal to a substantial fraction (less than unity) of the thickness (height, h) of the lens; whereas the term "fully recessed" refers to a situation where the surrounding layer of material has a height which is equal to or greater than that of the lens. However, the lenses made by the above-described method were not at all recessed.
Except for partially or fully recessing the lenses, the foregoing problems can be solved by using, instead of ion etching, a simultaneous gaseous chemical and energetic ion etching step, such as a reactive ion etching (RIE) process--i.e., by using an ion-assisted chemical etching technique using atomic or molecular radicals together with ions formed in a low pressure glow discharge--or by using a multiple beam etching process in which reactive chemical species are supplied by separate chemical beams as in CAIBE (chemically assisted ion beam etching). Thus, the problem of recessing remains, and hence it would be desirable to have a method for making lenses or microlenses in a substrate that are either partially or fully recessed from the substrate surface.